Abstract: An adjustably positionable element having a curved end plate is located in proximity to the conventional rupturable curved disc in a pressure relief valve for high pressure cylinders. The end plate serves as one plate of a capacitor and the curved disc serves as the other plate. Impedance variation responsive circuitry senses any change in impedance of the capacitor due to a change in flex of the curved disc brought about by a variation in pressure within the cylinder to which the relief valve is attached. The change in impedance is calibrated to provide an indication of the actual pressure within the cylinder.